MPLS WG
Internet Draft Matthew R. Meyer (Ed)
Global Crossing
Jean-Philippe Vasseur (Ed)
Cisco Systems, Inc
Denver Maddux
Nitrous.net
Curtis Villamizar
Amir Birjandi
Juniper Networks
Proposed status: Standard
Expires: November 2005 May 2005
MPLS Traffic Engineering Soft preemption
draft-ietf-mpls-soft-preemption-05.txt
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draft-ietf-mpls-soft-preemption-05.txt May 2005
Abstract
This document details MPLS Traffic Engineering Soft Preemption, a
suite of protocol modifications extending the concept of preemption
with the goal of reducing/eliminating traffic disruption of preempted
Traffic Engineering Label Switched Paths (TE LSPs). Initially MPLS
RSVP-TE was defined supporting only immediate TE LSP displacement
upon preemption. The utilization of a preemption pending flag helps
more gracefully mitigate the re-route process of preempted TE LSP.
For the brief period soft preemption is activated, reservations
(though not necessarily traffic levels) are in effect under-
provisioned until the TE LSP(s) can be re-routed. For this reason,
the feature is primarily but not exclusively interesting in MPLS
enabled IP networks with Differentiated Services and Traffic
Engineering capabilities.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [i].
Table of Contents
1. Terminology....................................................3
1.1 Acronyms and Abbreviations.................................3
1.2 Nomenclature...............................................3
2. Motivations....................................................4
3. Introduction...................................................4
4. RSVP Extensions................................................5
4.1 SESSION-ATTRIBUTES Flags...................................5
4.2 RRO IPv4/IPv6 Sub-Object Flags.............................5
4.3 Use of the RRO IPv4/IPv6 Sub-Object in Path message........5
5. Theory of Operation............................................6
6. Elements Of Procedures.........................................7
6.1 On a soft preempting LSR...................................7
6.2 On Head-end LSR of soft preempted TE LSP...................9
7. Interoperability...............................................9
8. Management....................................................10
9. IANA Considerations...........................................10
10. Security Considerations......................................10
11. Acknowledgments..............................................10
12. Intellectual Property Considerations.........................11
13. References...................................................11
13.1 Normative references.....................................11
13.2 Informative references...................................11
14. Authors' Addresses...........................................12
Meyer, Vasseur et al. [Page 2]
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1. Terminology
This document follows the nomenclature of the MPLS Architecture RFC
3031 [MPLS-ARCH].
1.1 Acronyms and Abbrevations
CSPF Constraint-based Shortest Path First.
DS Differentiated Services
LER Label Edge Router
LSR Label Switching Router
LSP Label Switched Path
MPLS MultiProtocol Label Switching
PPend Preemption Pending
RSVP Resource ReSerVation Protocol
TE Traffic Engineering
TE LSP Traffic Engineering Label Switched Path
1.2 Nomenclature
Make Before Break - Technique used to non-intrusively alter the path
of a TE LSP. The ingress LER first signals the new path, sharing the
bandwidth with the primary TE LSP (to avoid double booking), then
switches forwarding over to a new path. Finally the old path state is
torn down.
Numerically Lower Preemption Priority - TE LSPs have setup and hold
preemption priorities of zero (best) through seven (worst). A
numerically lower setup priority TE LSP is capable of preempting a
numerically higher hold priority TE LSP.
Preemption Pending flag - This flag is set on an IPv4 or IPv6 RSVP
Resv RRO sub-object to signal to the TE LSP ingress LER that the TE
LSP is about to be preempted and must be re-signaled (in a non
disruptive fashion, with make before break) along another path. If
present in the Path RRO, it is used to alert downstream LSRs that the
LSP was soft preempted upstream.
Point of Preemption - the midpoint or ingress LSR which due to RSVP
provisioning levels is forced to either hard preempt or under-
provision and signal soft preemption.
Hard Preemption - The (typically default) preemption process in which
higher numeric priority TE LSPs are intrusively displaced at the
point of preemption by lower numeric priority TE LSPs. In hard
preemption the TE LSP is torn down before reestablishment.
Soft Preemption - The preemption process in which the point of
preemption allows a brief under-provisioning period while the ingress
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router is alerted to the requirement for reroute. In soft preemption
the TE LSP is reestablished before being torn down.
Soft Preemption Desired Flag - This flag is set on the
SESSION_ATTRIBUTES Flags in the Path message for the TE LSP indicate
to LSRs along the path that, should the LSP need to be preempted,
soft preemption should be used if supported.
2. Motivations
Initially MPLS RSVP-TE [RSVP-TE] was defined supporting only one
method of TE LSP preemption which immediately tears down TE LSPs,
disregarding the preempted in-transit traffic. This simple but abrupt
process nearly guarantees preempted traffic will be discarded, if
only briefly, until the RSVP Path Error message reaches and is
processed by the ingress LER and a new forwarding path can be
established. In cases of actual resource contention this might be
helpful, however preemption may be triggered by mere reservation
contention and reservations may not reflect forwarding plane
contention up to the moment. The result is that when conditions that
promote preemption exist and hard preemption is the default behavior,
inferior priority preempted traffic may be needlessly discarded when
sufficient bandwidth exists for both the preempted LSP and the
preempting TE LSP(s).
Hard preemption may be a requirement to protect numerically lower
preemption priority traffic in a non Diff-Serv enabled architecture,
but in a Diff-Serv enabled architecture, one need not rely
exclusively upon preemption to enforce a preference for the most
valued traffic since the marking and queuing disciplines should
already be aligned for those purposes. Moreover, even in non Diff-
Serv aware networks, depending on the TE LSP sizing rules (imagine
all LSPs are sized at double their observed traffic level),
reservation contention may not accurately reflect the potential for
forwarding plane congestion.
3. Introduction
In an MPLS RSVP-TE [RSVP-TE] enabled IP network, hard preemption is
the default behavior. Hard preemption provides no mechanism to allow
preempted TE LSPs to be handled in a make-before-break fashion: the
hard preemption scheme instead utilizes a very intrusive method that
can cause traffic disruption for a potentially large amount of TE
LSPs. Without an alternative, network operators either accept this
limitation, or remove functionality by using only one preemption
priority or using invalid bandwidth reservation values.
Understandably desirable features like ingress LER automated TE
reservation adjustments are less palatable when preemption is
intrusive and high network stability levels are a concern.
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This document defines the use of additional signaling and maintenance
mechanisms to alert the ingress LER of the preemption that is pending
and allow for temporary under-provisioning while the preempted tunnel
is re-routed in a non disruptive fashion (make-before-break) by the
ingress LER. During the period that the tunnel is being re-routed,
link capacity is under-provisioned on the midpoint where preemption
initiated and potentially one or more links upstream along the path
where other soft preemptions may have occurred. Optionally the
downstream path to the egress LER may be signaled as well to more
efficiently deal with any near simultaneous soft preemptions that may
have been triggered downstream of the initial preemption.
4. RSVP Extensions
4.1 SESSION-ATTRIBUTES Flags
To explicitly signal the desire for a TE LSP to benefit from the soft
preemption mechanism (and so not to be hard preempted if the soft
preemption mechanism is available), the following flag of the
SESSION-ATTRIBUTE object (for both the C-Type 1 and 7) is defined:
Soft preemption desired: 0x40 (to be confirmed by IANA)
4.2 RRO IPv4/IPv6 Sub-Object Flags
To report that a soft preemption is pending for an LSP, a new flag is
defined in the IPv4/IPv6 sub-object carried in the RRO object message
defined in [RSVP-TE]. This flag is called the preemption pending
(PPend) flag. A compliant LSR MUST support the RRO object, as defined
in [RSVP-TE].
Several flags in the RRO IPv4 and IPv6 sub-object have been defined
in [RSVP-TE]and [FAST-REROUTE]:
This documents defines a new flag for the use of soft preemption
named the "Preemption pending" flag and defined as below:
Preemption pending: 0x10
The preempting node sets this flag if a pending preemption is in
progress for the TE LSP. This indicates to the ingress LER of this
LSP that it SHOULD be re-routed.
4.3 Use of the RRP IPv4/IPv6 Sub-Object in Path message
An LSR MAY use the Preemption pending flag in the IPv4/IPv6 RRO sub-
object carried in a PATH RRO message to simultaneously alert
downstream LSRs that the LSP was soft preempted upstream. This
information could be used by the downstream LSR to bias future soft
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preemption candidates toward LSPs already soft preempted elsewhere in
their path.
5. Theory of Operation
Let's consider the following example:
R0--1G--R1---155----R2 LSP1: LSP2:
| \ |
| \ 155 R0-->R1 R1<--R2
| \ | \ |
155 1G R3 V V
| \ | R5 R4
| \ 155
| \|
R4----1G----R5
Figure 1: example of Soft Preemption Operation
In the network depicted above in figure 1, consider the following
conditions:
-Reservable BW on R0-R1, R1-R5 and R4-R5 is 1Gb/sec.
-Reservable BW on R1-R2, R1-R4, R2-R3, R3-R5 is 155 Mb/sec.
-Bandwidths and costs are identical in both directions.
-Each circuit has an IGP metric of 10 and IGP metric is used by CSPF.
-Two TE tunnels are defined:
- LSP1: 155 Mb, setup/hold priority 0 tunnel, path R0-R1-R5.
- LSP2: 155 Mb, setup/hold priority 7 tunnel, path R2-R1-R4.
Both TE LSPs are signaled with the soft preemption desired bit of
their SESSION-ATTRIBUTE object set.
-Circuit R1-R5 fails.
-Soft Preemption is functional.
When the circuit R1-R5 fails, R1 detects the failure and sends an
updated IGP LSA/LSP and Path Error message to all the head-end LSRs
having a TE LSP traversing the failed link (R0 in the example above).
Either form of notification may arrive at the head-end LSRs first.
Upon receiving the link failure notification, R0 triggers a TE LSP
re-route of LSP1, and re-signals LSP1 along shortest path available
satisfying the TE LSP constraints: R0-R1-R4-R5 path. The Resv
messages for LSP1 travel in the upstream direction (from the
destination to the head-end LSR - R5 to R0 in this example). LSP2 is
soft preempted at R1 as it has a numerically lower priority value and
both bandwidth reservations cannot be satisfied on the R1-R4 link.
Instead of sending a path tear for LSP2 upon preemption as with hard
preemption (which would result in an immediate traffic disruption for
LSP2), R1s local bandwidth accounting for LSP2 is zeroed and a
preemption pending flagged Resv RRO for LSP2 is issued. Optionally,
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R1 MAY simultaneously send a soft preemption flagged Path RRO
notifying downstream LSRs of LSP2 soft preemption. If more than one
soft preempted LSP has the same head-end LSR R2 (egress LER), these
soft preemption Resv (Path) messages may be bundled together.
Upon reception of the LSP2's Resv message with the preemption pending
flag set, R2 may update the working copy of the TE-DB before running
CSPF for the new LSP. In the case that Diff-Serv [DIFF-MPLS] and TE
[RSVP-TE] are deployed, receiving preemption pending may imply to a
head-end LSR that the available bandwidth for the affected priority
level and numerically greater priority levels has been exhausted for
the indicated node interface. R2 may choose to reduce or zero
available bandwidth for the implied priority range until more
accurate information is available (i.e. a new IGP TE update is
received).
It follows that R2 re-computes a new path and performs a non traffic
disruptive rerouting of the new TE LSP T2 by means of the make-
before-break procedure. The old path is then torn down.
6. Elements Of Procedure
6.1 On a soft preempting LSR
When a new TE LSP is signaled which requires to preempt a set of TE
LSP(s) because not all TE LSPs can be accommodated on a specific
interface, a node triggers a preemption action which consists of
selecting the set of TE LSPs that must be preempted so as to free up
some bandwidth in order to satisfy the newly signaled numerically
lower preemption TE LSP.
For each preempted TE LSP, instead of sending a path tear upon
preemption as with hard preemption (which would result in an
immediate traffic disruption for the preempted TE LSP), the
preempting node's local bandwidth accounting for the preempted TE LSP
is zeroed and a preemption pending flagged Resv RRO for that TE LSP
is issued upstream toward the head-end LSR.
Optionally, the preempting node MAY simultaneously send a soft
preemption flagged Path RRO notifying downstream LSRs of soft
preemption. If more than one soft preempted TE LSP has the same
head-end LSR, these soft preemption Resv (Path) messages may be
bundled together.
The preempting node MUST immediately send a Resv message with the
preemption pending RRO flag set for each soft preempted TE LSP. The
node MAY use the occurrence of soft preemption to trigger an
immediate IGP update or influence the scheduling of an IGP update.
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Should a refresh event for a soft preempted TE LSP arrive before the
soft preemption timer expires, the soft preempting node MUST continue
to refresh the TE LSP.
When the MESSAGE-ID extensions defined in [REFRESH-REDUCTION] are
available, Resv messages with the RRO preemption pending flag set
SHOULD be sent in reliable mode.
In the case that reservation availability is restored at the point of
preemption, the point of preemption MAY issue a Resv message with the
preemption pending flag unset to signal restoration to the head-end
LSR. This implies that a head-end LSR might have delayed or been
unsuccessful in re-signaling.
To guard against a situation where bandwidth under-provisioning will
last forever, a local timer (named the 'Soft preemption timer') MUST
be started on the preemption node, upon soft preemption. If this
timer expires, the preempting node SHOULD send a PathTear and either
a ResvTear or a PathErr with the 'Path_State_Removed' flag set.
Selection of the preempted TE LSP at a preempting mid-point: when a
numerically lower priority TE LSP is signaled that requires the
preemption of a set of numerically higher priority LSPs, the node
where preemption is to occur has to make a decision on the set of TE
LSP(s), candidates for preemption. This decision is a local decision
and various algorithms can be used, depending on the objective. See
[PREEMPT-EXP]. As already mentioned, soft preemption causes a
temporary link under provisioning condition while the soft preempted
TE LSPs are rerouted by their respective head-end LSRs. In order to
reduce this under provisioning exposure, a soft-preempting LSR MAY
check first if there exists soft preempt-able TE LSP bandwidth
flagged PPend by another node but still available for soft-preemption
locally. If sufficient overlap bandwidth exists the LSR MAY attempt
to soft preempt the same LSP. This would help reducing the
temporarily elevated under-provisioning ratio on the links where soft
preemption occurs and the number of preempted TE LSPs. Optionally, a
midpoint LSR upstream or downstream from a soft preempting node MAY
choose to flag the LSPs soft preempted state. In the event a local
preemption is needed, the relevant priority level LSPs from the cache
are soft preempted first, followed by the normal soft and hard
preemption selection process for the given priority.
Under specific circumstances such as unacceptable link congestion, a
node MAY decide to hard preempt a TE LSP (by sending a PathTear and
either a ResvTear or a PathErr with the 'Path_State_Removed' flag
set) even if its head-end LSR explicitly requested 'soft preemption'
('Soft Preemption desired' flag of the corresponding SESSION-
ATTRIBUTE object set). Note that such decision MAY also be taken for
TE LSPs under soft preemption state.
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6.2 On the Head-end LSR of soft preempted TE LSP
Upon reception of a Resv message with the preemption pending flag
set, the head-end LSR MAY first update the working copy of the TE-DB
before computing a new path (e.g by running CSPF) for the new LSP. In
the case that Diff-Serv [DIFF-MPLS] and MPLS Traffic Engineering
[RSVP-TE] are deployed, receiving preemption pending may imply to a
head-end LSR that the available bandwidth for the affected priority
level and numerically greater priority levels has been exhausted for
the indicated node interface. A head-end LSR MAY choose to reduce or
zero available bandwidth for the implied priority range until more
accurate information is available (i.e. a new IGP TE update is
received).
Once a new path has been computed, the soft preempted TE LSP is
rerouted using the non traffic disruptive make-before-break
procedure.
As a result of soft preemption, no traffic will be needlessly black
holed due to mere reservation contention. If loss is to occur, it
will be due only to an actual traffic congestion scenario and
according to the operators Diff-Serv (if Diff-Serv is deployed) and
queuing scheme.
7. Interoperability
Backward compatibility should be assured as long as the
implementation followed the recommendations set forth in [RSVP-TE].
When processing an RRO, unrecognized sub-objects SHOULD be ignored
and passed on. An LSR without soft preemption capabilities but that
followed the aforementioned recommendation will simply ignore the RRO
Preemption Pending flag and treat the Resv message as a regular Resv
refresh message. As a consequence, the soft preempted TE LSP will not
be rerouted with make before break by the head-end LSR.
As mentioned prior, to guard against a situation where bandwidth
under-provisioning will last forever, a local timer (soft preemption
timer) MUST be started on the preemption node, upon soft preemption.
When this timer expires, the soft preempted TE LSP SHOULD be hard
preempted by sending a PathTear and either a ResvTear or a PathErr
with the 'Path_State_Removed' flag set. This timer SHOULD be
configurable and it is suggested to use a default value of 30
seconds.
The current hard preemption scheme can be emulated with a soft
preemption expiration timer set to zero. When set to 0, the TE LSP
SHOULD be hard-preempted.
Soft Preemption as defined in this document is designed for use in
MPLS RSVP-TE enabled IP Networks and may not functionally translate
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to some GMPLS technologies. As with backward compatibility, if a
device does not recognize a flag, it should pass the subobject
transparently.
8. Management
Both the point of preemption and the ingress LER SHOULD provide some
form of accounting internally and to the network operator interface
with regard to which TE LSPs and how much capacity is under-
provisioned due to soft preemption.
Displays of under-provisioning are recommended for the following
midpoint, ingress and egress views:
- Sum of current bandwidth per preemption priority per local
interface
- Sum of current bandwidth total per local interface
- Sum of current bandwidth total local router (ingress, egress,
midpoint)
- List current LSPs and bandwidth in PPend status
- List current sum bandwidth and session count in PPend status per
observed ERO hops (ingress, egress views only).
- Cumulative PPend events per observed ERO hops.
9. IANA Considerations
IANA [RFC-IANA] will not need to create a new registry. This document
requires the assignment of flags related to RFC3209 [RSVP-TE]
sections 4.1.1.1, 4.1.1.2, 4.7.1 and 4.7.2.
IANA will assign RRO IPv4/IPv6 sub-object flags defined in RFC3209
[RSVP-TE] sec 4.1.1.1 and 4.1.1.2 as detailed in section 4.2 of this
document.
IANA will assign session attribute flags for both the C-Type 1 and 7
(defined in RFC3209 [RSVP-TE] sec 4.7.1 and 4.7.2) as detailed in
section 4.1 of this document.
10. Security Considerations
This document does not introduce new security issues. The security
considerations pertaining to the original RSVP protocol [RSVP] remain
relevant.
11. Acknowledgments
The authors would like to thank Carol Iturralde, Dave Cooper, Loa
Andersson, Arthi Ayyangar, Ina Minei and George Swallow for their
valuable comments.
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12. Intellectual Property Considerations
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
13. References
13.1 Normative references
[RFC] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119.
[RFC-IANA] T. Narten and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434.
[MPLS-ARCH] Rosen, Viswanathan, Callon, "Multiprotocol Label
Switching Architecture", RFC3031, January 2001.
[RSVP] R. Braden, Ed., et al, "Resource ReSerVation protocol (RSVP) -
version 1 functional specification," RFC2205, September 1997.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC3209, December 2001.
13.2 Informative references
[TE-REQ] Awduche et al, Requirements for Traffic Engineering over
MPLS, RFC2702, September 1999.
[DS-TE] Le Faucheur et al, "Requirements for support of Diff-Serv-
aware MPLS Traffic Engineering", RFC3564, July 2003.
Meyer, Vasseur et al. [Page 11]
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[DS-TE-PROT] Le Faucheur et al, "Protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering", RFC3564, July 2003.
[REFRESH-REDUCTION] Berger et al, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[PREEMPT-EXP]De Oliviera, JP. Vasseur, L.Chen and C. Scoglio " LSP
Preemption Polcies for MPLS Traffic Engineering",
daft-deoliviera-diff-te-preemption-02.txt, October 2003
[DIFF-MPLS] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P. and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated Services",
RFC 3270, May 2002.
14. Authors' Addresses
Matthew R. Meyer
Global Crossing
3133 Indian Valley Tr.
Howell, MI 48855
USA
email: mrm@gblx.net, matthew.r.meyer@gmail.com
Denver Maddux
Nitrous.net
4237 E. Hartford Ave.
Phoenix, AZ 85032
USA
email: denver@nitrous.net
Jean-Philippe Vasseur
CISCO Systems, Inc.
300 Beaver Brook
Boxborough, MA 01719
USA
Email: jpv@cisco.com
Curtis Villamizar
AVICI
curtis@faster-light.net
Amir Birjandi
Juniper Networks
2251 corporate park dr ste
herndon, VA 20171
USA
abirjandi@juniper.net
Meyer, Vasseur et al. [Page 12]
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Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Meyer, Vasseur et al. [Page 13]
